Here is an introduction to “The Girl on the Train.” Listen carefully, and answer the questions that follow. Rachel ( Emily Blunt) used to be married to Tom ( Justin Theroux) , but Tom had an affair with Anna ( Rebecca Ferguson) , who is now his wife. He and Anna have a baby, whose nanny is named Megan ( Haley Bennett) . Megan looks a bit like Anna. She—Megan, not Anna—lives with Scott ( Luke Evans) , who is creepy and possessive, although Tom also looks a bit clenched. Not as clenched as Kamal ( Edgar Ramírez) , however, who is Megan’s superhot shrink. Rachel will later enroll as a patient of Kamal’s. Stay with me here. It so happens that Rachel, who is obsessed with her ex, takes a twice-daily train ride that passes the house where Tom and Anna live. One day, she—Rachel, not Anna—sees, or thinks she sees, a woman with blond hair, who could be Megan, although she might be mistaken for Anna, kissing a man with dark hair, who could be Scott, Tom, Kamal, or possibly the FedEx delivery guy, on a balcony. Faced with this devastating evidence, she, Rachel, becomes a sleuth, teaming up, slightly unwisely, with Scott, who believes, slightly wrongly, that she is a friend of Megan’s.